doxygen: enable AUTOBRIEF, drop \brief

Especially for short descriptions, it is annoying to have to type \brief for
every single API doc.

Drop all \brief and enable the AUTOBRIEF feature of doxygen, which always takes
the first sentence of an API doc as the brief description.

Change-Id: I11a8a821b065a128108641a2a63fb5a2b1916e87
diff --git a/include/osmocom/core/defs.h b/include/osmocom/core/defs.h
index 51ac4e5..88c41b9 100644
--- a/include/osmocom/core/defs.h
+++ b/include/osmocom/core/defs.h
@@ -5,10 +5,10 @@
  */
 
 /*! \file defs.h
- *  \brief General definitions that are meant to be included from header files.
+ *  General definitions that are meant to be included from header files.
  */
 
-/*! \brief Check for gcc and version.
+/*! Check for gcc and version.
  *
  * \note Albeit glibc provides a features.h file that contains a similar
  *       definition (__GNUC_PREREQ), this definition has been copied from there
@@ -23,7 +23,7 @@
 # define OSMO_GNUC_PREREQ(maj, min) 0
 #endif
 
-/*! \brief Set the deprecated attribute with a message.
+/*! Set the deprecated attribute with a message.
  */
 #if defined(__clang__)
 # define _OSMO_HAS_ATTRIBUTE_DEPRECATED __has_attribute(deprecated)